1. Field of Invention
The present invention relates to data processing used for example in full-color printing related equipment, such as printers, video printers, scanners, or the like, and image processing equipment such as those for generating computer-graphics images, or display devices such as monitors, and in particular to an image processing device and method for image-processing the image data represented by three colors of red, green and blue, or yellow, cyan and magenta, to be suitable for the equipment used.
2. Description of Related Art
In the field of printing, color conversion is an essential image technology for compensating for deterioration of printed images, due to color-mixing property caused by the fact that inks are not of pure colors, and due to non-linearity of the image printing, and to output printed images with a good color reproducibility. In display devices such as monitors, color conversion is effected as image processing for outputting (displaying) an image with a desired color reproduction characteristics to be suitable for the conditions in which the device is used, in displaying the input color data. The color conversion can be realized by a table conversion method or a matrix calculation method.
A representative example of table conversion method is a three-dimensional look-up table method, in which image data represented by red, green and blue (hereinafter referred to as “R, G, B”) are input, and image data of R, G, B, or complementary color data of yellow, magenta and cyan (hereinafter referred to as “Y, M and C” stored in advance in a memory, such as a ROM are output. This method permits use of any desired conversion characteristics so that color conversion with a good color reproducibility can be achieved.
A problem associated with the table conversion method is the size of the memory required. In a simple configuration in which, data to be output is stored for each combination of the input image data, a memory with a capacity of about 400 Mbits is needed. For instance, Japanese Patent Kokai Publication No. S63-227181 discloses a method of compressing the memory capacity, but the required capacity is still about 5 Mbits. Thus, this method requires a memory of a large capacity for each conversion characteristics, and it is difficult to implement the method by means of LSIs. Another problem is that the method cannot be flexibly adopted to changes in the use conditions or the like.
In the matrix calculation method, the following equation (1) is a basic calculation equation used for converting certain image data Ri, Gi, Bi to another image data Ro, Go, Bo.
                              [                                                    Ro                                                                    Go                                                                    Bo                                              ]                =                              (            Aij            )                    ⁡                      [                                                            Ri                                                                              Gi                                                                              Bi                                                      ]                                              (        1        )            
In the equation (1), i=1 to 3, j=1 to 3.
When color conversion is performed by the matrix calculation according to the equation (1), no large memory is required, so that the method can be implemented easily by means of LSIs, and the method can be adopted flexibly to the changes in the use conditions. On the other hand, the conversion characteristics which can be realized are limited by the matrix calculation equation, and the flexibility in the conversion characteristics is inferior to the three-dimensional look-up table method.
It will thus be understood that the table conversion method and the matrix calculation method used for color conversion have respective advantages and disadvantages. A problem common to both methods is that the number of expressed colors is reduced by the color conversion, and the discontinuities (irregularities) in the change in the data occur.
FIG. 27 is a block diagram showing an example of configuration of a conventional image processing device. In FIG. 27, reference numeral 1 denotes a color converter. The color converter 1 performs color conversion on input color data R1, G1, B1 constituting input image data, to calculate and produce output color data R3, G3, B3 constituting output image data. The color data R1, G1, B1 represent red, green and blue, respectively. The color data R3, G3, B3 also represent red, green and blue, respectively. The color conversion that is performed at the color converter 1 may be that of the table conversion method, or of the matrix calculation method, as described above. In the following description, it is assumed that in the image processing device shown in FIG. 27, the output color data R3, G3, B3 are calculated by a matrix calculation according to the following equation (2).
                              [                                                    R3                                                                    G3                                                                    B3                                              ]                =                              (            Aij            )                    ⁡                      [                                                            R1                                                                              G1                                                                              B1                                                      ]                                              (        2        )            
Here, it is assumed that each of the input color data R1, G1, B1, and the output color data R3, G3, B3 is six-bit data, and can express any value of 0 to 63. It is also assumed that each of the input color data R1, G1, B1 assumes all the values of 0 to 63, and the combination of the input color data R1, G1, B1 have 64*64*64=262,144 values. In other words, the input color data R1, G1, B1 can express 262,144 colors.
Let us assume that the matrix coefficients Aij used in the matrix calculation according to the equation (2) have values represented by the following equation (3).
                              (          Aij          )                =                  [                                                    1.0                                            0.0                                            0.0                                                                    0.0                                            1.0                                            0.0                                                                    0.2                                                              -                  0.2                                                            0.8                                              ]                                    (        3        )            
FIG. 28 shows the relationship between the input color data R1, G1, B1 input to the color converter 1, and the output color data R3, G3, B3 calculated by the color converter 1, for each of the nine values of the input color data (data No. 1 to No. 9), where-the matrix coefficients of the equation (3) are used. The nine values of input color data are such that the value of R1 is fixed at “40,” the value of G1is fixed at “32,” while the value of B1 is reduced from 40 to 32, one by one. In FIG. 28, Ra, Ga, Ba denote values of ideal output color data which would result if the output color data are not limited to six bits. Because the output color data R3, G3, B3 are in fact limited to six bits, the actual output color data R3, G3, B3 are those obtained by rounding Ra, Ga, Ba into six bit data. It is seen from FIG. 28 that the output color data R3, G3, B3 for the input color data R1=40, G1=32, B1=39, and the output color data R3, G3, B3 for the input color data R1=40, G1=32, B1=38 are both R3=40, G3=32, B3=32, i.e., of the same values. Also, the output color data R3, G3, B3 for the input color data R1=40, G1=32, B1=34, and the output color data R3, G3, B3 for the input color data R1=40, G1=32, B1=33 are both R3=40, G3=32, B3=28, i.e., of the same values.
As has been exemplified, a plurality of sets of input color data are converted to the same output color data, while the same input color data will not be converted to different sets of output color data, so that the number of colors that are expressed is reduced. That is, the number of colors expressed by the output color data R3, G3, B3 is smaller than 262,144.
FIG. 29 is a graph showing the relationship between the data numbers shown in FIG. 28, and the blue component B1 of the input color data, the value Ba of the blue component of the ideal output color data, and the blue component B3 of the output color data. It is seen from FIG. 29, that the blue component B1 of the input color data and the value Ba of the blue component of the ideal output color data vary smoothly, while the blue component B3 of the color data includes discontinuous (irregular) data change.
As has been described, the conventional image processing device and image processing method are associated with the problem that the number of colors that can be expressed is reduced by the color conversion. Moreover, the discontinuous (irregular) data change is generated by the color conversion. The color data output from the image processing device is supplied to an image display unit such as liquid crystal panel, and is displayed as an image. Thus, while the image display unit can express 262,144 colors, for instance, the number of colors expressed by the output color data R3, G3, B3 output from the image processing device is smaller than 262,144. This means that the capacity of the image display unit in terms of the number of colors which it can display cannot be fully utilized. Moreover, because of the discontinuity (irregularities) in the change of the data, the image will have parts where there is a larger or more sudden tone change and parts where there is a smaller or more gradual tone change, with the result that the reproduced image on the image display unit is not natural. In particular, if the an irregularity of the tone change occurs in the parts where the data varies gradually, a phenomenon called false edge by which an edge is seen as if it existed at a part where there is actually no edge.
The above described problems are more serious when the color data is changed to a larger extent by the color conversion at the color converter. There are different types of color reproduction, i.e., an “exact color reproduction” and a “preferred color reproduction.” The “exact color reproduction” is an exact or faithful color reproduction that is as close as possible to the original image. To realize the exact color reproduction, it is necessary to perform color reproduction which matches with the method of generation of the input image data. Specifically, it can be conceived to perform color reproduction using a standard color space such as those of NTSC or sRGB. On the other hand, a “preferred color reproduction” is a color reproduction that is preferred by a human being, taking account of the characteristics of the human visual perception, and memory colors, and is not necessary identical with the “exact color reproduction.”
In a color reproduction used for display of moving pictures, such as those of television pictures, a “preferred color reproduction” is often aimed at. In the memory colors of human beings, there is a tendency that the color of the sky, and the green color of the grass are memorized as colors which are brighter, or of a higher saturation. Accordingly, for realizing a “preferred color reproduction,” color conversion to increase the chroma and lightness of the colors is often applied to the input color data, so that the colors are changed to a greater extent. Moreover, even in the “exact color reproduction,” it is not rare that the colors are changed to a substantial extent. This is because the color gamut of the image display device is narrower than the color gamut of the color space used in the generation of the image data, or the standard color space.
The conventional image processing device or image processing method is thus associated with a problem that the number of colors expressed is reduced by the color conversion, and that discontinuities (irregularities) in the change of data are created by the color conversion. Since the color data output from the image processing device are input to an image display unit such as a liquid crystal panel, and an image corresponding to the color data is displayed, the reduction in the number of the colors expressed means that the capacity of the image display unit in terms of the number of colors which it can display is not fully utilized. Moreover, because of the discontinuities (irregularities) in the change of the data, there will be parts where the change in the tone is large or sudden, and the parts where the change in the tone is small or gradual, with the result that the reproduced image displayed on the image display unit may be unnatural.